Repbase Update (http://www.girinst.org), is a comprehensive collection of DNA sequences, currently representing over 1200 families and subfamilies of repetitive elements from all eukaryotic species. Each family is identifiable by a unique name, referenced to published literature and often characterized in brief commentaries. Several hundreds of repetitive families added to Repbase Update during the last two years are either unreported anywhere else, or have been thoroughly revised in terms of their consensus sequences and biological classification. Our approach is based on computer-assisted exploration and analysis of publicly available DNA sequence data. Repbase Update has been routinely used by public institutions and private companies throughout the world in routine gene discovery, sequence assembly and probe design. In addition, Repbase Update became a unique resource for individual research projects of biological and medical importance. In light of the anticipated flood of new sequence data, we propose continuation of development, maintenance and electronic dissemination of this resource. In particular, we expect to discover and characterize hundreds of new repetitive families which will continue to be published electronically in Repbase Update. Furthermore, we will continue to compile and organize reported families of repeats from all eukaryotic species. Finally, we will maintain the public server for automatic annotation of human and rodent repeats and generate up-to-date annotation of repetitive elements present in genomic DNA of mammals. In particular we propose to: 1. Continue development, maintenance and electronic distribution of reference collections of repetitive elements from all sequenced eukaryotic species. 2. Generate public domain maps of repetitive elements present in available mammalian genomic DNA sequences.